## 2021-12-06 - Lanternfish

(original .ipynb)

Day 6 puzzle input is a sequence of integers representing the ages of lanternfish (mine is here). Part 1 involves following the lifecycle of these lanternfish over **80** days, where every 7 days lanternfish will spawn a new one (so a lanternfish that starts with age `4`

will produce a new fish in `4`

days, then another `7`

days after that). Part 2 involves doing this over **256** days.

puzzle_input_str = open("puzzle_input/day6.txt").read() test_input_str = """3,4,3,1,2""" def part_one(input_str, days=80): fish = [int(age) for age in input_str.split(",")] while days > 0: new_fish = 0 for i in range(len(fish)): if fish[i] == 0: fish[i] = 6 new_fish += 1 else: fish[i] -= 1 fish += [8] * new_fish days -= 1 return len(fish) assert 5934 == part_one(test_input_str) print("part one:", part_one(puzzle_input_str))

part one: 356190

# part_one(puzzle_input_str, 256) <--- with 102 steps remaining the naive algo reports # that there are 244,225,750 fish which suggests # I do not have enough memory to keep this going # # so after seeing the O(N^2) algo won't run in reasonable time my heart sunk and I realised # I might actually have to think about this one. first thought: could I maybe keep the # existing brute force algo and chunk it into 12 processes and just run it on my 12 core # cpu at home ... then when trying to break the problem down like that realised the order # of the fish doesn't matter - just need to count how many there are at each step # # we are not our best selves at 6:00am def part_two(input_str): fish = [0] * 9 for age in input_str.split(","): fish[int(age)] += 1 days = 256 while days > 0: next_fish = [0] * 9 for age, count in enumerate(fish): if age == 0: next_fish[6] += count next_fish[8] += count else: next_fish[age-1] += count fish = next_fish days -= 1 return sum(fish) assert 26984457539 == part_two(test_input_str) print("part two:", part_two(puzzle_input_str))

part two: 1617359101538